Ru II-III Complexes

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

Mixed valency occurs in minerals (e.g. 63 ), metal-chain compounds, dimers and oligomers and metal complexes, and even in organic and biological systems (Brown, 1980 Day, 1981). Among the dimeric and oligomeric metal complexes exhibiting mixed valency, the pyrazine-bridged Ru (II, III) ammine complex. 

To illustrate the tuning aspects of the MLCT transitions in ruthenium polypyridyl complexes, let us begin by considering the well-known ruthenium mT-bipyridine complex (1). Complex 1 shows strong visible band at 466 nm, due to charge-transfer transition from metal t2g (HOMO) orbitals to tt orbitals (LUMO) of the ligand. The Ru(II)/(III) oxidation potential is at 1.3 V, and the ligand-based reduction potential is at -1.5 V versus SCE [36]. From spectro chemical and electrochemical studies of polypyridyl complexes of ruthenium, it has been con-... 

Os(bpy)2LL]2+/1+/0 (L-L are quinone, semiquinone, or catecholato ligands derived from catechol, 3,5-di-ter<-butylcatechol, or tetrachlo-rocatechol) have been characterized by UV/Vis/NIR and EPR spectroscopies. These spectroscopic properties and the crystal structure of [0s(bpy)2(dbcat)]C104 confirm an Os( III (-catecholate ground state for the +1 ion. This contrasts with the ground state of the +1 ions of Ru analogs, which are best described as Ru(II)-semiquinone complexes... 

Ru(II) halosulfoxide complexes catalyse the oxidation of secondary alcohols by N-methylmorpholi nc-/ -ox idc (NMO) via a proposed Ru(IV)oxo species.92 Ruthenium (VI) catalyses the oxidation of diethylene glycol by alkaline solution of potassium bromate.93 Acid bromate oxidation of butylethylene glycol is catalysed by ruthenium(III).94 Ruthenium(III) catalyses DMF oxidation by periodate in alkaline... 

Further examples of energy transfer processes in assemblies involving Ir(III) polyimine derivatives, from Williams et al., are shown in Fig. 25b and c [133]. These are bimetallic assemblies containing Ir and Ru centers, either an Ir(terpy)23+ connected to a Ru(bpy)32+ via a polyphenyl bridge, Fig. 25b, or a Ru(terpy)22+ connected to an Ir(ppy)2bpy+ moiety, Fig. 25c. Energy transfer occurs in both arrays in the direction Ir->- Ru, but whereas for (b), Fig. 25, the sensitized luminescence of the Ru center upon excitation of the Ir center can be detected also at ambient temperature, in array (c) the sensitized luminescence of the Ru(II)-terpyridine complex can only be revealed at low temperatures (165 K), due to the well-known non-emissive nature of Ru(terpy)22+ at room temperature. 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

Activation of Ru(III)hexammine in CO atmosphere At activation temperatures below 373 K, Ru(III)hexammine in the presence of CO is transformed into a Ru(II)pentammine complex. The identification of this complex is based on earlier work (7, 8) and the information is summarized in Table 4. This transformation is almost quantitative and the material formed shows already a low WGS-activity. Chemically, this transformation can be visualized as follows (8) ... 

Dinuclear ruthenium complexes form the largest group by far of any mixed-valence system and are the exclusive subject of this chapter. Ruthenium is the transition metal of choice to study electron transfer or exchange because it is relatively inexpensive and forms stable Ru(III) and Ru(II) coordination complexes. In addition, the synthetic coordination chemistry of ruthenium is well developed (1). 

Dyads and triads based on the photoactive, multibridging [Ru(bpz)3] (bpz = bipyrazine) complex directly bound to transition metal complexes were obtained by following the procedures previously reported for the generation of symmetric heptanuclear supermolecules (67-69). Such systems contain a tris(bpz)ruthenium (II) ion [RuJ attached to bis(bpy)chlororuthenium(II)/(III) [Rup], or penta-cyanoferrate(II)/(III) complexes via a bpz bridging ligand, as shown for the... 

Data on complex formation constants -log k were taken from Irving and Williams (1953), MoeUer et al. (1965), Izatt et al. (1971), Euria (1972), Kiss et al. (1991), and Mizerski (1997), plus values scattered elsewhere in the literature, while E (L) values are derived from Lever (1990). This means that a potential (-shift) scale based on the Ru(II/III) redox couple is used rather than the older ones from the Chatt workgroup which draw upon Mo(0/I)-, Mn(I/II)- and similar low-valent couples (it should be pointed out that these concerning trani-[Mo" (dppe)j(Nj)L] with L = Hal F to I, ChCN- Ch=O, S or Se, N, CO, PR3, RCN, CS(NH ), etc. and dppe = 1,2-bis-diphenylphosphinoethane can be linked to Lever s scale by linear correlation with very high correlation coefficients (Franzle 2005, unpublished)). The index nd (denticity n) corresponds to the... 

Chiral (salen)Cr(III) have also been used (entry 3) [196] and have found application in the total synthesis of muconin (see also Sch. 16) [197], This reaction is also catalyzed by irradiation of a chiral (salen)Ru(II)(NO) complex in the presence of the diene and the hetero-dienophile (entry 4) [198],... 

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